It started as a lovely drive down Malibu's Pacific Coast Highway. Jennifer Aniston and Justin Theroux were on their way back to Bel Air after visiting Robert Downey Jr., his wife and their new baby. But by the time they turned their Mercedes sedan onto Sunset Boulevard, the jealous fighting had started.

Why? On the heels of John Mayer's recent love song about Jen — and her subsequent texts with the singer, she is set to pair up with former fling Jason Sudeikis in the upcoming comedy We're the Millers.

But Justin isn't laughing. He had been biting his tongue all day," a source close to the couple tells OK!. "But he couldn't keep his mouth shut for one more second." Justin actually pulled the car over, turned off the engine, looked straight into Jen's eyes and asked, "Why are you doing this to me?" says the source.